Fast steering mirrors (“FSMs”) have become key components in diverse applications spanning from high value space assets to tactical military hardware and industrial instrumentation. FSMs range in size from a few millimeters to a half meter and typically only move 2-3° in any direction. An FSM can be used to perform a variety of functions including tracking, scanning, pointing, line of sight stabilization, and alignment. FSMs are used for image motion control to stare at a scene while the platform moves and then rapidly reposition the line of sight during the focal plane readout period. In an image motion control application, the FSM will be operable to continuously correct for environmental disturbances such as air turbulence or vehicle vibrations. FSMs are also used in laser pointing applications, which require accurately controlled point-to-point movements. FSMs are often a part of space telescopes. FSMs are also a key component of military aircraft infrared countermeasure pods used to protect aircraft from missile attacks during takeoff and landing.
Traditional steering mirrors, specifically FSMs, include four voice coil actuators to provide fast, high-bandwidth movement. Utilized in push-pull pairs, the actuators provide smooth, even torque to the mirror. Current is provided to the voice coils to tip and tilt the mirror assembly. An internal electromagnetic or optical-based feedback loop is built into the FSM head (one or more sensor pairs per rotation axis) to provide position feedback with reference to the support frame for accurate and stable pointing and tracking. However, the sensors often have electric fields that can be affected by the actuator magnetic flux causing interference to the measured angles.
Typically, steering mirror actuators use a coil of wire attached to the steering mirror base, and a magnet attached to the moving mirror. Some geometry of soft magnetic material is attached to the magnet to direct the magnetic flux perpendicular to the coil walls so that the Lorentz force equation (F=I*B) can be used to estimate the force. The result for a typical voice coil type actuator is a large path length through air for the magnetic flux which reduces the magnetic flux density as described by Gauss' law for magnetism applied to the surfaces of the magnet. Very small air gap path lengths, and much larger flux densities, are possible by using soft magnetic materials to direct the magnetic flux from the magnet faces through the center of wire coils. The commonly used materials are typically lousy with the rapidly changing flux density that are necessary to achieve fast response times, which can lead to excessive phase loss for a stable control system and compounds problems with actuator heat.
Over the years, steering mirrors have improved due to advances in the optics, the actuators that move the mirror, the feedback sensors that determine mirror position, and the control system that ties everything together. However, existing FSMs have reached an upper limit of fast step response without producing a lot of excess heat due to the rapidly changing flux density. The excess heat requires additional mass and volume for thermal management and can distort components reducing the accuracy of the steering mirror. Additionally, alternative actuators, such as stepper or brushless servo motors (linear and rotary), are too large for application with FSMs and suffer from the same limitations described above.